The application relates generally to automotive vehicles, and more particularly to vehicles with automatic stop/start systems.
In motor vehicles with automatic stop/start systems, an internal combustion engine can be automatically stopped, i.e. switched off, when no propulsion power is required (for example, when waiting at a traffic light), and automatically restarted, i.e. using a starter, when the driver again requests propulsion power. Such stop/start operation reduces fuel consumption and noise emissions.
In motor vehicles with vacuum brake force boosters and automatic stop/start systems, the negative pressure prevailing in the brake force booster must be monitored in order to ensure that sufficient auxiliary braking force is always available. When the internal combustion engine is not running, no negative pressure is generated. If the driver operates the brake in this situation the negative pressure decreases rapidly to a value at which the auxiliary braking force is insufficient. This is undesirable, primarily for safety reasons.
To restart the internal combustion engine in a timely manner, a negative pressure sensor may be provided on the brake force booster and the actual negative pressure value may be compared to a threshold value. The internal combustion engine can be restarted when the negative pressure value falls below the threshold value. Such a sensor, however, leads to additional hardware cost and complexity.
Another simple alternative would be to count the actuations of the brake pedal and compare them to a threshold value. This solution may, however, be imprecise.
It would be highly desirable to have a precise and cost-effective method for estimating negative pressure in a motor vehicle brake force booster with reference to hydraulic pressure in a brake master cylinder.